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The principle of capacitive sensing is not a new one. The technique relies on the fact that any object in free space has an electrical capacitance to earth. The value of this capacitance depends on several factors, the principle ones being the object's surface area, the distance to earth and the permitivity of the intervening medium.

The range and sensitivity of an instrument depends on the ability of the electronics to accurately measure extremely small values of capacitance. It is in this area that much of Sensatech's research has been focused, resulting in techniques which maximise noise rejection and minimise the unwanted effects of temperature.

Sensatech has developed specific and unique skills focused around areas critical to the development and production of complex sensing solutions:

Sensor Head Design:
Array control:
Software development:
Amplifier design:
EIT Electrical impedance tomography:
Remote sensing:
Basic types of electric field sensing:
Look or signal through optically opaque targets:
Look around targets:

1 Sensor Head Design:

Sensing electrodes are designed using field modelling techniques to create ideal sensing electrodes to match specific applications. Sense electrodes can be optimised to retain very low parasitic capacitance which allows multiplexing of multiple inputs from many electrodes (such as in an array) with minimum signal degradation and the detection of very small changes in the target.

2 Array control:

Arrays of multiple sensor heads are multiplexed to allow Sensatech to collect large sets of data and build images of a target object which can then be analysed with proprietary embedded software. Because through careful electrode design, parasitic capacitance remains very small, Sensatech are able to measure down to 3 fempto farad total capacitance with 12 bit resolution and 3 atto farrad accuracy. Such small change is valuable for example in gap measurement. This low parasitic capacitance also permits positioning sensor electrodes at the end of co-ax cables separate from amplifier and electronics. These remote sensor heads, sometimes multiples of separate heads, can then operate in hazardous environments.

3 Software development:

Data sets generated by sensor arrays can be large and complex. Some arrays contain 2000 odd electrodes all of which are scanned and the resulting data once processed provides knowledge of the target object.

Software can be taught the characteristics of the particular sensor in order to identify and reduce noise and interference. Often reference capacitors are included in the PCB design to further calibrate sensor elements.

Sensatech analytical software can vary from simple to very complex depending on the task requirements:

  • Neural nets can be used to predict what a sensor should see if detecting particular targets.
  • Finite element analysis can split the target space into many sub spaces each of which can be separately solved to produce an overall result,
  • A sensitivity analysis database normally goes with each sensor to aid its resolution of particular target families. In the example of the foot-gauge the software copes with feet presented at widely varying angles and is programmed to ignore targets that do not resemble human feet.

4 Amplifier design.

Low noise with high input impedence and low capacitance guard amplifiers have been built with excellent temperature stability.

5 Electrical impedance tomography

Uses array generated data, then using back propogation techniques compares and reshapes the permiability characteristics of the space until it meets the data found by the sensor. This is the basis of X-RAY type tomography that Sensatech carry out using electric fields.

6 Remote sensing:

15m away from the amplifier and signal electronics.

7 Basic types of electric field sensing:

  • Standard electric field sensing: Transmit and receive: In this format a transmitter produces a field and if the capacitance between the transmit and receive plates is large the sensor detects this.
  • Standard using an array: A more complex variant of the standard electric field sensing model is to use an array of transmit and receive plates which generates many fields. By AC driving the plates at a frequency for example less than 1 Megahertz this provides a large data set. Using a swept frequency it is even possible to determine chemical parameters of the target material or object. This can be used for example for height independent soil water content.
  • Potential sensing: Because Sensatech receiver plates have only a few fimto farads of parasitic capacitance, if a voltage exists on an object, that voltage will be creating a field and this can be sensed at a range.
    E.g. Sensatech can detect the potential created by a heart beat 1cm away from the surface of the skin;
    For mine detector use Sensatech set up a voltage gradient in the soil, with other electrodes and sense this gradient with a large matrix above the soil. Any metal or plastic in the soil distorts the voltage gradient and this distortion can be measured to determine what is inside the soil. Normally such work is in conjunction with a normal transmit and receive sensing method.
  • EIT Electrical Impedance Tomography: While EIT is normally carried out with a transmitter and receiver plate in contact with the target object, using the techniques outlined above, Sensatech do not require to contact or invade the target object and can non invasively detect through other materials.
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